It is known to disable a vehicle occupant protection device, such as an air bag for an occupant of a vehicle seat, if it is determined that an occupant of the seat is under a certain weight. There are various known seat weight sensing systems that determine the weight or load on the vehicle seat. These known seat weight sensing systems typically include weight sensors that are mounted on the vehicle seat. It is also known to distinguish between the weight of an adult seated in the seat, of a child seated in the seat, and of a child seat cinched down tight in the vehicle seat and to permit, inhibit, or tailor actuation of the protection device in response thereto.
The “zero point” of a seat weight sensing system relates to the weight sensed by the system when there is no load on the seat. The seat weight sensor of the seat weight sensing system has a zero load output when there is no load on the seat. The seat weight sensing system is calibrated to associate a weight of zero with this zero load output. Over time, the zero load output of the seat weight sensor may vary due to factors, such as drift in the seat weight sensor, fatigue in the seat, or fatigue in other vehicle structures. As a result, the seat weight sensing system may read a non-zero weight when there is no load on the seat. This is referred to as a zero point error.
It is known to adjust a zero point in a seat weight sensing system to compensate for a zero point error that falls within a predetermined range. For example, the predetermined range may be −4 kg to +4 kg. In one known method, the system detects a zero point error when sensed conditions indicate that the seat is likely to be unloaded. For example, the system may detect an unloaded seat if the following conditions are satisfied: the sensed weight is within the small range (e.g., −4 kg to +4 kg), the seatbelt is unlatched, the vehicle ignition has remained un-actuated for a predetermined period of time, and there have been no weight fluctuations for a predetermined period of time (e.g., 15 minutes). If a zero point error is detected, the system adjusts the zero point of the system to help correct the error. This can be done by adjusting the zero point to correct the entire error, a portion (e.g., half) of the error, or a portion of the error up to a maximum value (e.g., up to 1 kg).
A problem may occur where the seat weight sensor has a negative zero point error within the predetermined range and an object having a weight outside the correctable range is placed on the seat. For example, if a zero point error causes the system to sense −1.5 kg and a 5 kg object, such as a briefcase, is placed on the seat, the sensed weight would go positive to +3.5 kg, which is within the correctable range. If the object is left on the seat for an extended period, the system may, over time, correct the zero point with the object on the seat. When the object is removed, the sensor would read negative (−5 kg), which is outside the correctable range. This may result in no subsequent zero-point adjustments taking place.